Coronary arterial 18F-FDG uptake by fusion of PET and coronary CT angiography at sites of percutaneous stenting for acute myocardial infarction and stable coronary artery disease

[18F]FDG Accumulation in Early Coronary Atherosclerotic Lesions in Pigs

Objective

Inflammation is an important contributor to atherosclerosis progression. A glucose analogue ¹⁸F-fluorodeoxyglucose ([¹⁸F]FDG) has been used to detect atherosclerotic inflammation. However, it is not known to what extent [¹⁸F]FDG is taken up in different stages of atherosclerosis. We aimed to study the uptake of [¹⁸F]FDG to various stages of coronary plaques in a pig model.

Methods

First, diabetes was caused by streptozotocin injections (50 mg/kg for 3 days) in farm pigs (n = 10). After 6 months on high-fat diet, pigs underwent dual-gated cardiac PET/CT to measure [¹⁸F]FDG uptake in coronary arteries. Coronary segments (n = 33) were harvested for ex vivo measurement of radioactivity and autoradiography (ARG).

Results

Intimal thickening was observed in 16 segments and atheroma type plaques in 10 segments. Compared with the normal vessel wall, ARG showed 1.7±0.7 times higher [¹⁸F]FDG accumulation in the intimal thickening and 4.1±2.3 times higher in the atheromas (P = 0.004 and P = 0.003, respectively). Ex vivo mean vessel-to-blood ratio was higher in segments with atheroma than those without atherosclerosis (2.6±1.2 vs. 1.3±0.7, P = 0.04). In vivo PET imaging showed the highest target-to-background ratio (TBR) of 2.7. However, maximum TBR was not significantly different in segments without atherosclerosis (1.1±0.5) and either intimal thickening (1.2±0.4, P = 1.0) or atheroma (1.6±0.6, P = 0.4).

Conclusions

We found increased uptake of [¹⁸F]FDG in coronary atherosclerotic lesions in a pig model. However, uptake in these early stage lesions was not detectable with in vivo PET imaging. Further studies are needed to clarify whether visible [¹⁸F]FDG uptake in coronary arteries represents more advanced, highly inflamed plaques.

Inflammation, Atherosclerosis, and Coronary Artery Disease: PET/CT for the Evaluation of Atherosclerosis and Inflammation

Atherosclerosis is a prevalent cardiovascular disease marked by inflammation and the formation of plaque within arterial walls. As the disease progresses, there is an increased risk of major cardiovascular events. Owing to the nature of atherosclerosis, it is imperative to develop methods to further understand the physiological implications and progression of the disease. The combination of positron emission tomography (PET)/computed tomography (CT) has proven to be promising for the evaluation of atherosclerotic plaques and inflammation within the vessel walls. The utilization of the radiopharmaceutical tracer, 18F-fluorodeoxyglucose (¹⁸F-FDG), with PET/CT is invaluable in understanding the pathophysiological state involved in atherosclerosis. In this review, we will discuss the use of ¹⁸F-FDG-PET/CT imaging for the evaluation of atherosclerosis and inflammation both in preclinical and clinical studies. The potential of more specific novel tracers will be discussed. Finally, we will touch on the potential benefits of using the newly introduced combined PET/magnetic resonance imaging (MRI) for non-invasive imaging of atherosclerosis.

Non-invasive imaging for subclinical coronary atherosclerosis in patients with peripheral artery disease

Patients with peripheral artery disease are at high risk of coronary artery disease. An increasing number of studies show that a large proportion of patients with peripheral artery disease have significant coronary atherosclerosis, even in the absence of symptoms. Although the reported prevalence of subclinical coronary artery disease varies widely in patients with peripheral artery disease, it could include more than half of patients. No consensus exists to date on either the rationale for screening patients with peripheral artery disease for coronary atherosclerosis or the optimal algorithm and method for screening. An increasing number of imaging modalities are emerging that allow improved in vivo non-invasive characterization of atherosclerotic plaques. These novel imaging methods may lead to early detection of high-risk vulnerable plaques, enabling clinicians to improve risk stratification of patients with peripheral artery disease, and thus paving the way for individualized therapy.

[¹⁸F]-fluorodeoxyglucose PET imaging of atherosclerosis

[(18)F]-fluorodeoxyglucose PET ((18)FDG PET) imaging has emerged as a promising tool for assessment of atherosclerosis. By targeting atherosclerotic plaque glycolysis, a marker for plaque inflammation and hypoxia, (18)FDG PET can assess plaque vulnerability and potentially predict risk of atherosclerosis-related disease, such as stroke and myocardial infarction. With excellent reproducibility, (18)FDG PET can be a surrogate end point in clinical drug trials, improving trial efficiency. This article summarizes key findings in the literature, discusses limitations of (18)FDG PET imaging of atherosclerosis, and reports recommendations to optimize imaging protocols.

NIH workshop on clinical translation of molecular imaging probes and technology--meeting report

A workshop on "Clinical Translation of Molecular Imaging Probes and Technology" was held August 2, 2013 in Bethesda, Maryland, organized and supported by the National Institute of Biomedical Imaging and Bioengineering (NIBIB). This workshop brought together researchers, clinicians, representatives from pharmaceutical companies, molecular probe developers, and regulatory science experts. Attendees met to talk over current challenges in the discovery, validation, and translation of molecular imaging (MI) probes for key clinical applications. Participants also discussed potential strategies to address these challenges. The workshop consisted of 4 sessions, with 14 presentations and 2 panel discussions. Topics of discussion included (1) challenges and opportunities for clinical research and patient care, (2) advances in molecular probe design, (3) current approaches used by industry and pharmaceutical companies, and (4) clinical translation of MI probes. In the presentations and discussions, there were general agreement that while the barriers for validation and translation of MI probes remain high, there are pressing clinical needs and development opportunities for targets in cardiovascular, cancer, endocrine, neurological, and inflammatory diseases. The strengths of different imaging modalities, and the synergy of multimodality imaging, were highlighted. Participants also underscored the continuing need for close interactions and collaborations between academic and industrial partners, and federal agencies in the imaging probe development process.

Targeting post-infarct inflammation by PET imaging: comparison of (68)Ga-citrate and (68)Ga-DOTATATE with (18)F-FDG in a mouse model

Imaging of inflammation early after myocardial infarction (MI) is a promising approach to the guidance of novel molecular interventions that support endogenous healing processes. (18)F-FDG PET has been used, but may be complicated by physiological myocyte uptake. We evaluated the potential of two alternative imaging targets: lactoferrin binding by (68)Ga-citrate and somatostatin receptor binding by (68)Ga-DOTATATE.

METHODS

C57Bl/6 mice underwent permanent coronary artery ligation. Serial PET imaging was performed 3 - 7 days after MI using (68)Ga-citrate, (68)Ga-DOTATATE, or (18)F-FDG with ketamine/xylazine suppression of myocyte glucose uptake. Myocardial perfusion was evaluated by (13)N-ammonia PET and cardiac geometry by contrast-enhanced ECG-gated CT.

RESULTS

Mice exhibited a perfusion defect of 30 - 40% (of the total left ventricle) with apical anterolateral wall akinesia and thinning on day 7 after MI. (18)F-FDG with ketamine/xylazine suppression demonstrated distinct uptake in the infarct region, as well as in the border zone and remote myocardium. The myocardial standardized uptake value in MI mice was significantly higher than in healthy mice under ketamine/xylazine anaesthesia (1.9 ± 0.4 vs. 1.0 ± 0.1). (68)Ga images exhibited high blood pool activity with no specific myocardial uptake up to 90 min after injection (tissue-to-blood contrast 0.9). (68)Ga-DOTATATE was rapidly cleared from the blood, but myocardial SUV was very low (0.10 ± 0.03).

CONCLUSION

Neither (68)Ga nor (68)Ga-DOTATATE is a useful alternative to (18)F-FDG for PET imaging of myocardial inflammation after MI in mice. Among the three tested approaches, (18)F-FDG with ketamine/xylazine suppression of cardiomyocyte uptake remains the most practical imaging marker of post-infarct inflammation.

The role of PET quantification in cardiovascular imaging

Positron Emission Tomography (PET) has several clinical and research applications in cardiovascular imaging. Myocardial perfusion imaging with PET allows accurate global and regional measurements of myocardial perfusion, myocardial blood flow and function at stress and rest in one exam. Simultaneous assessment of function and perfusion by PET with quantitative software is currently the routine practice. Combination of ejection fraction reserve with perfusion information may improve the identification of severe disease. The myocardial viability can be estimated by quantitative comparison of fluorodeoxyglucose (¹⁸FDG) and rest perfusion imaging. The myocardial blood flow and coronary flow reserve measurements are becoming routinely included in the clinical assessment due to enhanced dynamic imaging capabilities of the latest PET/CT scanners. Absolute flow measurements allow evaluation of the coronary microvascular dysfunction and provide additional prognostic and diagnostic information for coronary disease. Standard quantitative approaches to compute myocardial blood flow from kinetic PET data in automated and rapid fashion have been developed for ¹³N-ammonia, ¹⁵O-water and ⁸²Rb radiotracers. The agreement between software methods available for such analysis is excellent. Relative quantification of ⁸²Rb PET myocardial perfusion, based on comparisons to normal databases, demonstrates high performance for the detection of obstructive coronary disease. New tracers, such as ¹⁸F-flurpiridaz may allow further improvements in the disease detection. Computerized analysis of perfusion at stress and rest reduces the variability of the assessment as compared to visual analysis. PET quantification can be enhanced by precise coregistration with CT angiography. In emerging clinical applications, the potential to identify vulnerable plaques by quantification of atherosclerotic plaque uptake of ¹⁸FDG and ¹⁸F-sodium fluoride tracers in carotids, aorta and coronary arteries has been demonstrated.

A randomized trial on the optimization of 18F-FDG myocardial uptake suppression: implications for vulnerable coronary plaque imaging

(18)F-FDG PET/CT can be used to detect arterial atherosclerotic plaque inflammation. However, avid myocardial glucose uptake may preclude its use for visualizing coronary plaques. Fatty acid loading or calcium channel blockers could decrease myocardial (18)F-FDG uptake, thus assisting coronary plaque inflammation identification. The present prospective randomized trial compared the efficacies of different interventions for suppressing myocardial (18)F-FDG uptake. We also investigated whether circulating free fatty acid (cFFA) levels predicted the magnitude of myocardial (18)F-FDG uptake.

METHODS

Thirty-six volunteers ate a high-fat low-carbohydrate meal, followed by a 12-h fasting period. They were then randomized to 1 of 4 intervention groups. Group 1 received no additional preparation and served as a reference. Groups 2 and 3, respectively, received a commercial high-fat solution containing 43.8 g of lipids or 50 mL of olive oil 1 h before (18)F-FDG injection to evaluate the impact of fatty acid loading on myocardial (18)F-FDG uptake. Group 4 received verapamil to evaluate the effect of calcium channel blockers. Cardiac PET/CT was performed after administration of 370 MBq of (18)F-FDG. Myocardial uptake suppression was assessed using a qualitative visual scale and by measuring the myocardial maximum standardized uptake value (SUV(max)). Insulin, glucose, and cFFA were serially measured.

RESULTS

The qualitative visual scale showed good myocardial (18)F-FDG uptake suppression in 8 of 9, 5 of 9, 4 of 9, and 8 of 9 subjects of groups 1, 2, 3, and 4, respectively (P = 0.09). SUV(max) did not significantly differ between groups (P = 0.17). Interestingly, cFFA levels were higher in volunteers with good suppression (0.80 ± 0.31 mmol/L) than in those with poor suppression (0.53 ± 0.15 mmol/L; P = 0.011). We found an inverse correlation between cFFA level (measured at (18)F-FDG injection) and the SUV(max) (R = 0.61). Receiver-operating-characteristic curve analysis identified 0.65 mmol/L cFFA as the best cutoff value to predict adequate (18)F-FDG uptake suppression (positive predictive value, 89%).

CONCLUSION

A high-fat low-carbohydrate meal followed by a 12-h fasting period effectively suppressed myocardial (18)F-FDG uptake in most subjects. Neither complementary fatty acid loading nor verapamil administered 1 h before (18)F-FDG injection conferred any additional benefit. Myocardial (18)F-FDG uptake was inversely correlated with cFFA level, representing an interesting way to predict myocardial (18)F-FDG uptake suppression.

Periaortic brown adipose tissue as a major determinant of [¹⁸F]-fluorodeoxyglucose vascular uptake in atherosclerosis-prone, apoE-/- mice

BACKGROUND

[18F]-fluorodeoxyglucose (FDG) has been suggested for the clinical and experimental imaging of inflammatory atherosclerotic lesions. Significant FDG uptake in brown adipose tissue (BAT) has been observed both in humans and mice. The objective of the present study was to investigate the influence of periaortic BAT on apolipoprotein E-deficient (apoE-/-) mouse atherosclerotic lesion imaging with FDG.

METHODS

ApoE-/- mice (36 ± 2 weeks-old) were injected with FDG (12 ± 2 MBq). Control animals (Group A, n = 7) were injected conscious and kept awake at room temperature (24°C) throughout the accumulation period. In order to minimize tracer activity in periaortic BAT, Group B (n = 7) and C (n = 6) animals were injected under anaesthesia at 37°C and Group C animals were additionally pre-treated with propranolol. PET/CT acquisitions were performed prior to animal euthanasia and ex vivo analysis of FDG biodistribution.

RESULTS

Autoradiographic imaging indicated higher FDG uptake in atherosclerotic lesions than in the normal aortic wall (all groups, P<0.05) and the blood (all groups, P<0.01) which correlated with macrophage infiltration (R = 0.47; P<0.001). However, periaortic BAT uptake was either significantly higher (Group A, P<0.05) or similar (Group B and C, P = NS) to that observed in atherosclerotic lesions and was shown to correlate with in vivo quantified aortic FDG activity.

CONCLUSION

Periaortic BAT FDG uptake was identified as a confounding factor while using FDG for the non-invasive imaging of mouse atherosclerotic lesions.

Pioglitazone decreases coronary artery inflammation in impaired glucose tolerance and diabetes mellitus: evaluation by FDG-PET/CT imaging

OBJECTIVES

The aim of this study was to compare the effect of pioglitazone with glimepiride on coronary arterial inflammation with serial (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) combined with computed tomography (CT) angiography.

BACKGROUND

Recent studies have shown that FDG-PET combined with CT is a reliable tool to visualize and quantify vascular inflammation. Although pioglitazone significantly prevented the progression of coronary atherosclerosis and reduced the recurrence of myocardial infarction in patients with type 2 diabetes mellitus (DM), it remains unclear whether pioglitazone could attenuate coronary artery inflammation.

METHODS

Fifty atherosclerotic patients with impaired glucose tolerance or type 2 DM underwent determination of blood chemistries, anthropometric and inflammatory variables, and FDG-PET/CT angiography, and then were randomized to receive either pioglitazone or glimepiride for 16 weeks. Effects of the treatments on vascular inflammation of the left main trunk were evaluated by FDG-PET/CT angiography at baseline and end of the study. Vascular inflammation of the left main trunk was measured by blood-normalized standardized uptake value, known as a target-to-background ratio.

RESULTS

Three patients dropped out of the study during the assessment or treatment. Finally, 25 pioglitazone-treated patients and 22 glimepiride-treated patients (37 men; mean age: 68.1 ± 8.3 years; glycosylated hemoglobin: 6.72 ± 0.70%) completed the study. After 16-week treatments, fasting plasma glucose and glycosylated hemoglobin values were comparably reduced in both groups. Changes in target-to-background ratio values from baseline were significantly greater in the pioglitazone group than in the glimepiride group (-0.12 ± 0.06 vs. 0.09 ± 0.07, p = 0.032), as well as changes in high-sensitivity C-reactive protein (pioglitazone vs. glimepiride group: median: -0.24 [interquartile range (IQR): -1.58 to -0.04] mg/l vs. 0.08 [IQR: -0.07 to 0.79] mg/l, p = 0.031).

CONCLUSIONS

Our study indicated that pioglitazone attenuated left main trunk inflammation in patients with impaired glucose tolerance or DM in a glucose-lowering independent manner, suggesting that pioglitazone may protect against cardiac events in patients with impaired glucose tolerance or DM by suppressing coronary inflammation. (Anti-Inflammatory Effects of Pioglitazone; NCT00722631).

Imaging RAGE expression in atherosclerotic plaques in hyperlipidemic pigs

Background

Receptor for advanced glycated end product (RAGE) expression is a prominent feature of atherosclerosis. We have previously shown in apoE null mice uptake of a radiolabeled anti-RAGE antibody in atherosclerotic plaque and now evaluate RAGE-directed imaging to identify advanced plaques in a large animal model.

Methods

Nine hyperlipidemic (HL) pigs were injected with 603.1 ± 129.5 MBq of ^99mTc-anti-RAGE F(ab′)₂, and after 6 h (blood pool clearance), they underwent single-photon emission computed tomography/computed tomography (SPECT/CT) imaging of the neck, thorax, and hind limbs. Two HL pigs received ^99mTc non-immune IgG F(ab′)₂, and three farm pigs were injected with ^99mTc-anti-RAGE F(ab′)₂. After imaging, the pigs were euthanized. The aorta from the root to bifurcation was dissected, and the innominates, proximal carotids, and coronaries were dissected and counted, stained for H&E and RAGE, and AHA-classified.

Results

On pathology, 24% of the arterial segments showed AHA class III or IV lesions, and these lesions were confined almost exclusively to coronaries and carotids with % stenosis from 15% to 65%. Scatter plots of %ID/g for class III/IV vs. I/II lesions showed almost complete separation. Focal vascular uptake of tracer visualized on SPECT scans corresponded to class III/IV lesions in the coronary and carotid vessels. In addition, uptake in the hind limbs was noted in the HL pigs and corresponded to RAGE staining of small arteries in the muscle sections. Correlations for the vascular lesions were r = 0.747, P = 0.001 for %ID vs. %ID/g and r = 0.83, P = 0.002 for %ID/g vs. % RAGE staining.

Conclusions

Uptake of radiolabeled anti-RAGE antibody in coronary and carotid fibroatheroma and in the small arteries of the hind limbs in a relevant large animal model of atherosclerosis supports the important role of RAGE in atherosclerosis and peripheral artery disease as a target for imaging and treatment.

Cardiovascular implantable electronic device infection: delayed vs standard FDG PET-CT imaging

BACKGROUND

Positron emission tomography-computed tomography (PET-CT) with (18)F-fluorodeoxyglucose (FDG) has emerged as a rapidly evolving diagnostic tool for infectious diseases. However, the optimal imaging time in this clinical setting is not clear yet. The aim of this study is to investigate whether delayed (3 hours) FDG PET-CT could increase the diagnostic accuracy of this technique compared to standard (1 hour) imaging in the detection of septic foci involving the pocket and/or pacing leads in patients with suspected cardiovascular implantable electronic device (CIED) infection scheduled for device removal.

METHODS AND RESULTS

Twenty-seven patients underwent standard and delayed imaging. PET-CT results were compared to bacteriological cultures after CIED removal. Fifteen controls free of infection underwent PET-CT imaging as part of investigation of malignancy. The diagnostic accuracy of delayed imaging was significantly higher than 1-hour scan for lead infection (70% vs 51%, P = .024). No significant difference was found between standard and delayed diagnostic accuracy for pocket or device infection. Semi-quantitative analysis showed that mean pocket and lead target-to-background ratio were significantly higher on delayed compared to standard imaging (3.7 ± 1.9 vs 1.6 ± 1.1, P = .0002; 3.0 ± 1.3 vs 0.7 ± 1.0, P = .01).

CONCLUSIONS

Delayed FDG PET-CT imaging should be considered at least in patients with negative 1-hour scan and founded suspicion of pacing lead infection.

Imaging atherosclerosis and risk of plaque rupture

Atherosclerosis imaging strategies can delineate characteristics of plaques at risk of rupture and thrombosis. Structural plaque imaging identifies high-risk plaque features, including lipid pools, thin fibrous caps, and intraplaque hemorrhage. New molecular imaging techniques complement structural imaging approaches by illuminating important features of plaque biology, with a prominent focus on detecting inflammation as a high-risk phenotype. As we unravel the molecular and structural characteristics underlying thrombosis-prone plaques, there is significant promise for eventual early identification and prediction of atherosclerotic plaque complications before they occur. Here we focus on recent imaging insights into high-risk arterial plaques, the etiologic agent of acute myocardial infarction, stroke, and sudden cardiac death.

18F-fluoride positron emission tomography for identification of ruptured and high-risk coronary atherosclerotic plaques: a prospective clinical trial

BACKGROUND

The use of non-invasive imaging to identify ruptured or high-risk coronary atherosclerotic plaques would represent a major clinical advance for prevention and treatment of coronary artery disease. We used combined PET and CT to identify ruptured and high-risk atherosclerotic plaques using the radioactive tracers (18)F-sodium fluoride ((18)F-NaF) and (18)F-fluorodeoxyglucose ((18)F-FDG).

METHODS

In this prospective clinical trial, patients with myocardial infarction (n=40) and stable angina (n=40) underwent (18)F-NaF and (18)F-FDG PET-CT, and invasive coronary angiography. (18)F-NaF uptake was compared with histology in carotid endarterectomy specimens from patients with symptomatic carotid disease, and with intravascular ultrasound in patients with stable angina. The primary endpoint was the comparison of (18)F-fluoride tissue-to-background ratios of culprit and non-culprit coronary plaques of patients with acute myocardial infarction.

FINDINGS

In 37 (93%) patients with myocardial infarction, the highest coronary (18)F-NaF uptake was seen in the culprit plaque (median maximum tissue-to-background ratio: culprit 1·66 [IQR 1·40-2·25] vs highest non-culprit 1·24 [1·06-1·38], p<0·0001). By contrast, coronary (18)F-FDG uptake was commonly obscured by myocardial uptake and where discernible, there were no differences between culprit and non-culprit plaques (1·71 [1·40-2·13] vs 1·58 [1·28-2·01], p=0·34). Marked (18)F-NaF uptake occurred at the site of all carotid plaque ruptures and was associated with histological evidence of active calcification, macrophage infiltration, apoptosis, and necrosis. 18 (45%) patients with stable angina had plaques with focal (18)F-NaF uptake (maximum tissue-to-background ratio 1·90 [IQR 1·61-2·17]) that were associated with more high-risk features on intravascular ultrasound than those without uptake: positive remodelling (remodelling index 1·12 [1·09-1·19] vs 1·01 [0·94-1·06]; p=0·0004), microcalcification (73% vs 21%, p=0·002), and necrotic core (25% [21-29] vs 18% [14-22], p=0·001).

INTERPRETATION

(18)F-NaF PET-CT is the first non-invasive imaging method to identify and localise ruptured and high-risk coronary plaque. Future studies are needed to establish whether this method can improve the management and treatment of patients with coronary artery disease.

FUNDING

Chief Scientist Office Scotland and British Heart Foundation.

Efficient radiosynthesis of 3'-deoxy-3'-18F-fluorothymidine using electrowetting-on-dielectric digital microfluidic chip

Access to diverse PET tracers for preclinical and clinical research remains a major obstacle to research in cancer and other disease research. The prohibitive cost and limited availability of tracers could be alleviated by microfluidic radiosynthesis technologies combined with a high-yield microscale radiosynthetic method. In this report, we demonstrate the multistep synthesis of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) with high yield on an electrowetting-on-dielectric (EWOD) microfluidic radiosynthesizer, previously developed in our group. We have identified and established several parameters that are most critical in the microscale radiosynthesis, such as the reaction time, reagent concentration, and molar ratios, to successfully synthesize (18)F-FLT in this compact platform.

METHODS

(18)F-FLT was synthesized from the 3-N-Boc-1-[5-O-(4,4'-dimethoxytrityl)-3-O-nosyl-2-deoxy-β-D-lyxofuranosyl] thymine precursor on the EWOD chip starting from the first solvent exchange and (18)F-fluoride ion activation step to the final deprotection step. The fluorination reaction was performed in a mixture of thexyl alcohol and dimethyl sulfoxide. The crude product after deprotection was collected from the chip and purified on a custom-made solid-phase extraction cartridge and subjected to quality control testing. The purified (18)F-FLT was suitable for small-animal PET studies in multiple nude mice xenografted with the A431 carcinoma cell line.

RESULTS

(18)F-FLT was successfully synthesized on the EWOD microdevice coupled with an off-chip solid-phase extraction purification with a decayed-corrected radiochemical yield of 63% ± 5% (n = 5) and passed all of the quality control tests required by the U.S. Pharmacopeia for radiotracers to be injected into humans. We have successfully demonstrated the synthesis of several batches of (18)F-FLT on EWOD, starting with approximately 333 MBq of radioactivity and obtained up to 52 MBq (non-decay-corrected) of (18)F-FLT on cartridge purification. The specific activity of 2 representative preparations of (18)F-FLT synthesized on the EWOD chip were measured to be 1,800 and 2,400 GBq/μmol.

CONCLUSION

The EWOD microchip and optimized synthesis method in combination represent an effective platform for synthesizing (18)F-FLT with high yield and of good quality for imaging. This compact platform, with configurable synthesis steps, could potentially form the basis of a stand-alone system that decouples PET probe production from the cyclotron and specialized radiochemistry facilities and increases diversity and flexibility in probe production.

Recent advances in metabolic imaging

Abnormalities in myocardial substrate metabolism play a central role in the manifestations of most forms of cardiac disease such as ischemic heart disease, heart failure, hypertensive heart disease, and the cardiomyopathy due to either obesity or diabetes mellitus. Their importance is exemplified by both the development of numerous imaging tools designed to detect the specific metabolic perturbations or signatures related to these different diseases, and the vigorous efforts in drug discovery/development targeting various aspects of myocardial metabolism. Since the prior review in 2005, we have gained new insights into how perturbations in myocardial metabolism contribute to various forms of cardiac disease. For example, the application of advanced molecular biologic techniques and the development of elegant genetic models have highlighted the pleiotropic actions of cellular metabolism on energy transfer, signal transduction, cardiac growth, gene expression, and viability. In parallel, there have been significant advances in instrumentation, radiopharmaceutical design, and small animal imaging, which now permit a near completion of the translational pathway linking in-vitro measurements of metabolism with the human condition. In this review, most of the key advances in metabolic imaging will be described, their contribution to cardiovascular research highlighted, and potential new clinical applications proposed.

Erroneous cardiac ECG-gated PET list-mode trigger events can be retrospectively identified and replaced by an offline reprocessing approach: first results in rodents

The assessment of left ventricular function, wall motion and myocardial viability using electrocardiogram (ECG)-gated [(18)F]-FDG positron emission tomography (PET) is widely accepted in human and in preclinical small animal studies. The nonterminal and noninvasive approach permits repeated in vivo evaluations of the same animal, facilitating the assessment of temporal changes in disease or therapy response. Although well established, gated small animal PET studies can contain erroneous gating information, which may yield to blurred images and false estimation of functional parameters. In this work, we present quantitative and visual quality control (QC) methods to evaluate the accuracy of trigger events in PET list-mode and physiological data. Left ventricular functional analysis is performed to quantify the effect of gating errors on the end-systolic and end-diastolic volumes, and on the ejection fraction (EF). We aim to recover the cardiac functional parameters by the application of the commonly established heart rate filter approach using fixed ranges based on a standardized population. In addition, we propose a fully reprocessing approach which retrospectively replaces the gating information of the PET list-mode file with appropriate list-mode decoding and encoding software. The signal of a simultaneously acquired ECG is processed using standard MATLAB vector functions, which can be individually adapted to reliably detect the R-peaks. Finally, the new trigger events are inserted into the PET list-mode file. A population of 30 mice with various health statuses was analyzed and standard cardiac parameters such as mean heart rate (119 ms ± 11.8 ms) and mean heart rate variability (1.7 ms ± 3.4 ms) derived. These standard parameter ranges were taken into account in the QC methods to select a group of nine optimal gated and a group of eight sub-optimal gated [(18)F]-FDG PET scans of mice from our archive. From the list-mode files of the optimal gated group, we randomly deleted various fractions (5% to 60%) of contained trigger events to generate a corrupted group. The filter approach was capable to correct the corrupted group and yield functional parameters with no significant difference to the optimal gated group. We successfully demonstrated the potential of the fully reprocessing approach by applying it to the sub-optimal group, where the functional parameters were significantly improved after reprocessing (mean EF from 41% ± 16% to 60% ± 13%). When applied to the optimal gated group the fully reprocessing approach did not alter the functional parameters significantly (mean EF from 64% ± 8% to 64 ± 7%). This work presents methods to determine and quantify erroneous gating in small animal gated [(18)F]-FDG PET scans. We demonstrate the importance of a quality check for cardiac triggering contained in PET list-mode data and the benefit of optionally reprocessing the fully recorded physiological information to retrospectively modify or fully replace the cardiac triggering in PET list-mode data. We aim to provide a preliminary guideline of how to proceed in the presence of errors and demonstrate that offline reprocessing by filtering erroneous trigger events and retrospective gating by ECG processing is feasible. Future work will focus on the extension by additional QC methods, which may exploit the amplitude of trigger events and ECG signal by means of pattern recognition. Furthermore, we aim to transfer the proposed QC methods and the fully reprocessing approach to human myocardial PET/CT.

Biological imaging of atherosclerosis: moving beyond anatomy

Biological or molecular imaging is now providing exciting new strategies to study atherosclerosis in both animals and humans. These technologies hold the promise to provide disease-specific, molecular information within the context of a systemic or organ-specific disease beyond traditional anatomical-based imaging. By integration of biological, chemical, and anatomical imaging knowledge into diagnostic strategies, a more comprehensive and predictive picture of atherosclerosis is likely to emerge. As such, biological imaging is well positioned to study different stages of atherosclerosis and its treatment, including the sequence of atheroma initiation, progression, and plaque rupture. In this review, we describe the evolving concepts in atherosclerosis imaging with a focus on coronary artery disease, and we provide an overview of recent exciting translational developments in biological imaging. The illuminated examples and discussions will highlight how biological imaging is providing new clinical approaches to identify high-risk plaques, and to streamline the development process of new atherosclerosis therapies.